1. Field of the Invention
The present invention relates to a process for recovering a tungsten compound by burning a waste liquor containing a soluble tungsten compound, such as a distillation residue.
Heretofore, tungstic acid, its alkali salt or compounds containing tungsten as a component, such as heteropoly acid, have been used as catalysts for epoxidizing or hydroxylating a water-soluble, double bond-containing compound such as allyl alcohol. Especially, tungsten compounds are excellent as catalysts for epoxidation reactions. Because they are expensive, these catalysts are an important factor for the economy of the production process, so that a great number of processes for recovering tungsten-containing catalysts have been proposed.
The present invention provides an advantageous process for recovering a catalyst component in a effective form by burning a waste liquor as mentioned above to gasify organic components.
2. Description of the Prior Art
Although processes for recovering a tungsten compound include a precipitation process comprising the addition of calcium chloride, an ion exchange process with the use of an ion exchange resin, and a precipitation process with the use of a strong alkali or a strong acid, these processes have a drawback that waste water highly loaded with salts is produced, so that the quantitative recovery of catalysts is not possible.
Because tungstic acid or its alkali salt is relatively unstable to heat and partially forms tungsten oxide at high temperatures, it undergoes decreases in water solubility and catalytic activity. Therefore, a process wherein burning is performed in a fluidized bed at a relatively low temperature below the melting point (Japanese Patent Publications No. 43136/1983 and No. 43142/1983) is practically the only process known as a process for treating a waste liquor containing a compound of this kind by burning. However, because of the characteristics of the burning process, it is difficult to gasify the organic compounds completely in a single-stage fluidized bed, and therefore carbonaceous substances remain almost inevitably. In order to remove the carbonaceous substances completely, it is necessary to prolong the residence time or to use a multi-stage bed, but in any case, a larger apparatus and additional fuel are required, which is not economical. When the burning temperature is increased in the case of a low-melting compound (for example, Na.sub.2 WO.sub.4, m.p. 698.degree. C.), not only is its fluidization difficult, but also the recovery of the tungsten oxide formed by decomposition due to prolonged heating is extremely difficult, because tungsten oxide is easily sublimable to form a fume. On the other hand, it has been believed that spray burning is impossible because a considerable portion of a tungsten compound is insolubilized in water under usual conditions for a waste water, i.e., a temperature of 800.degree. to 1,000.degree. C. and a residence time of about 1 second, and this causes a marked decrease in the catalyst activity.